build: Use AC_SUBST instead of AC_DEFINE for BUILD_DEVEL_DOCS

debian/changelog

@@ -41,6 +41,8 @@ dpkg (1.18.5) UNRELEASED; urgency=medium
     - Only warn once when a diff patches a file multiple times in
       Dpkg::Source::Patch, and fix the warning message to make it clear that
       the diff might be patchig the file more than once, not just twice.
+  * Build system:
+    - Fix building development documentation.
   * Test suite:
     - Add a unit test to compile perl code with warnings.
   * Documentation:

m4/dpkg-build.m4

@@ -40,9 +40,9 @@ AC_DEFUN([DPKG_BUILD_DEVEL_DOCS], [
     [build_devel_docs=yes]
   )
   AS_IF([test "x$build_devel_docs" = "xyes"], [
-    AC_DEFINE([BUILD_DEVEL_DOCS], [YES], [Define to YES if building devel docs.])
+    AC_SUBST([BUILD_DEVEL_DOCS], [YES])
   ], [
-    AC_DEFINE([BUILD_DEVEL_DOCS], [NO])
+    AC_SUBST([BUILD_DEVEL_DOCS], [NO])
   ])
 ])# DPKG_BUILD_DOCS_MODE